For the detection of extraterrestrial life from extrasolar planets, the identification of potential biosignatures as gaseous molecules such as oxygen and surface spectral features originating from life is the most promising way. In the latter case, the vegetation red edge (VRE), which is the steep difference in the vegetation reflectance spectrum, has been conventionally considered. However, predicting the wavelength at which the edge of "exovegetation" appears would be difficult, e.g., around M dwarf stars, which are the current targets of observations and whose radiation is far from that of the Sun. Here we present the possible detection of photosynthetic fluorescence as a biosignature in addition to VRE, based on a recent paper of ours summarized by an interdisciplinary discussion from biology to astronomy. The fluorescence has recently been detected by remote sensing of the Earth. We investigated how the signal appears in the planetary spectrum. Photoabsorption, reflection, and fluorescence are consistently modeled because they are physically related. A mock observation of an Earth-Sun system at 10 pc from a future space telescope shows the enormously long exposure time required to identify the fluorescence. However, around ultracool dwarfs such as
TRAPPIST
-1, which has strong stellar absorption of VO and FeH, the apparent reflection of an Earth-like planet with fluorescence whose spectrum is that of bacteriochlorophyll
b-bearing purple bacteria was greatly enhanced, which could be a promising signal for high-dispersion spectroscopy using future ground-based telescopes such as TMT. Possible false positive/negative detection of the fluorescence would be better eliminated by simultaneous detection with the VRE and/or nonlinearity of the photosynthetic fluorescence.
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